The immune system is quite complex and includes many different pathways for an organism to fight infectious pathogens and cancer cells. In general, the immune system is viewed as being able to mount a humoral immune response (HIR) and/or a cell-mediated immune response (CMI). The HIR involves the production and secretion of antibodies produced in the cells of the B lymphocyte lineage (B-cells). Secreted antibodies bind to antigens on the surfaces of invading microbes (such as viruses or bacteria). The antibody-bound antigens are then destroyed by various cells in the immune system. Humoral immunity also refers to antibody production and the accessory processes that accompany it. It also refers to the effector functions of antibody, which include pathogen and toxin neutralization, classical complement activation, and opsonin promotion of phagocytosis and pathogen elimination.
The second type of immune response is cell-mediated immunity (CMI). CMI is an immune response that does not involve antibodies or complement but instead involves the activation of various immune cells, such as macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen. Cellular immunity can protect the body by activating antigen-specific T-lymphocytes. These cells induce apoptosis in body cells displaying epitopes of foreign antigen on their surface, such as virus-infected cells, cells infected with intracellular bacteria, and cancer cells displaying tumor antigens. T cells activate macrophages and natural killer cells, enabling them to destroy intracellular pathogens, and stimulating cells to secrete a variety of cytokines that influence the function of other cells involved in adaptive and innate immune responses. Cell-mediated immunity is directed primarily at microbes that survive in phagocytes and microbes that infect non-phagocytic cells. It is most effective in removing virus-infected cells, but also participates in defending against fungi, protozoans, cancers, and intracellular bacteria.
Traditionally, as defined by the World Health Organization, a vaccine is any preparation intended to produce immunity to a disease by stimulating the production of antibodies. Vaccines include, for example, suspensions of killed or attenuated microorganisms, or products or derivatives of microorganisms. The most common method of administering vaccines is by inoculation, but some are given by mouth or nasal spray.
Present vaccine technologies rely on large doses of antigen and/or re-vaccinations (booster shots) and do not confer protection against all infectious agents. Therefore, a need exists for new vaccines to confer protection against infectious agents for which there currently are no effective vaccines. There is also a need for new vaccines that are safer to administer, are less expensive to produce, and/or do not require booster shots. The elimination of booster shots would increase immunization compliance. Finally, some human populations, such as the elderly, make overall weaker responses to vaccination, and more effective vaccines could better protect this growing category of vaccine recipients.